Lithium batteries are becoming increasingly popular in the market place as they offer advantages of high voltage and energy density over conventional aqueous technologies. Recent developments in the field are allowing lithium battery technologies to meet the demands of ever greater energy density requirements for electronics applications.
Primary lithium batteries and historically secondary lithium batteries have used metallic lithium or a lithium alloy for the anode and often a transition metal chalcogenide for the corresponding cathode. The discharge process of batteries constructed in this way involves the transfer of lithium atoms from the anode into the host cathode. Thus, primary cells preferably have minimal lithium in the cathode as fabricated in order to obtain maximum capacity on discharge. Certain advantages however can be obtained by partially lithiating the cathode prior to battery fabrication. Common commercial cathode materials include manganese oxide compounds. As taught in the Hitachi Maxell KK, Japanese patent publication 59-31182 (1984), discharge performance and shelf stability of a non-aqueous Li.vertline.MnO.sub.2 primary cell can be improved by partial lithiation of the MnO.sub.2 cathode followed by appropriate heat treatment. This was accomplished by soaking the MnO.sub.2 in LiOH solution and then heat treating preferably around 300.degree. C. The MnO.sub.2 contains significant amounts of bound water which is driven off by heating. In this publication, it is stated that the presence of lithium in the host MnO.sub.2 prevents undesirable changes on heating.
The prior art contains other references teaching similar ways of lithiating manganese oxides for use in batteries. Sanyo, Japanese Kokai 62-108455 (1987) disclose a secondary battery employing cathode of lithium doped gamma phase electrolytic manganese dioxide made in the same general way described in the Hitachi '182 publication. Toshiba, Japanese Kokai 62-126556 (1987) describe batteries made with cathode material prepared from LiOH treated MnO.sub.2. Moli, U.S. Pat. No. 4,959,282, describe the preparation of what was called "X-phase" cathode material for batteries where the lithiated manganese oxide is first coated with LiOH solid via evaporation from solution followed by a heat treatment step. In all the preceding examples, only partial lithiation of the manganese compound is performed. Further, in the latter example, the lithiation is predominantly achieved via a solid state reaction.
In U.S. Pat. No. 5,166,012, Roussouw et al. disclose preparation methods for certain lithiated manganese oxide compounds wherein more substantial lithiation is achieved. In particular, a compound having a hollandite structure with stoichiometry H.sub.2x-z Li.sub.z MnO.sub.2+x wherein z might be as great as 0.6 is disclosed. As is commonly known in the art, Roussouw confirms that some amount (undefined) of lithiation can be expected to occur via the partial ion exchange reaction of lithium for hydrogen in the starting hydrogen manganese oxide reactant. However, those of ordinary skill in the art will realize that the lithiation is predominantly accomplished via a conventional solid state reaction. The presence of water during the synthesis and hence even the conditions necessary for ion exchange are not essential to the disclosed methods. Thus, while ion exchange certainly can take place in the preparation of H.sub.2x-z Li.sub.z MnO.sub.2+x, lithiation occurs predominantly via a solid state reaction. Accordingly, the amount of lithium which is ion exchanged must be, at most, a small fraction of the total lithium content, z.
Recent developments in the field have led to the commercialization of rechargeable lithium batteries where a host compound, usually a form of carbon, has been used in place of lithium metal and/or alloy as the anode. During use of the battery, lithium ions are shuttled or rocked from the cathode to the anode on charge and from the anode to the cathode on discharge. Such batteries are called Lithium ion cells (T. Nagaura and K. Tozawa, Progress in Batteries and Solar Cells, 9, 209, (1990)) or Rocking Chair cells (J. R. Dahn, et al., J. Electrochem. Soc. 138, 2207 (1991)) Such batteries provide increased safety and cycle life over historical rechargeable lithium technologies.
It is desirable for practical reasons that the battery components be relatively stable in dry air for manufacturing purposes. It is also desirable that substantial amounts of lithium be incorporated in the electrode materials such that use of the inherent capacity of the materials can be maximized without having to add lithium in some other form. Lithium atoms therefore usually reside in the transition metal chalcogenide cathode when fabricated as the preferred compounds for use can incorporate large amounts of lithium yet still remain stable in dry air.
Sony Energy Tec Inc. was the first company to commercialize lithium ion batteries where LiCoO.sub.2 was used as a cathode material. Many other such materials have been proposed such as LiNiO.sub.2 (Goodenough et al., U.S. Pat. No. 4,302,518 and/or Dahn et al., J. Electrochem. Soc. 138, 2207, (1991)) LiMn.sub.2 O.sub.4 (Ohzuku et al., J. Electrochem. Soc. 137, 769, (1990)) and Li.sub.2 Mn.sub.2 O.sub.4 (Tarascon et al., J. Electrochem. 138, 2864 (1991)). LiCoO.sub.2 and LiNiO.sub.2 adopt a layered structure of space group R-3m. LiMn.sub.2 O.sub.4 adopts the AB.sub.2 O.sub.4 spinel structure with space group Fd3m. Li.sub.2 Mn.sub.2 O.sub.4 as described in Tarascon et al., J. Electrochem. 138, 2864 (1991) is related to LiMn.sub.2 O.sub.4 in structure, but it is not believed to be entirely air stable. It is our belief that said Li.sub.2 Mn.sub.2 O.sub.4 is hence not useful as a practical cathode material for Lithium ion batteries.
Each of these cathode materials can reversibly react with a certain amount of lithium between reasonable cutoff voltages that might be used in a practical Lithium-ion cell. These cutoff voltages are most likely about 2.5 V and about 4.2 V versus metallic Li. The specific capacities of LiCoO.sub.2, LiNiO.sub.2 and LiMn.sub.2 O.sub.4 between these voltage limits are about 140 mAh/g (J. N. Reimers and J. R. Dahn, J. Electrochem. Soc. 139, 2091 (1992) ), 170 mAh/g, (J. R. Dahn, U. von Sacken and C. A. Michael, Solid State Ionics 44, 87 (1990)), and 110 mAh/g (T. Ohzuku, M. Kitagawa and T. Hirai, J. Electrochem. Soc. 137, 769, (1990)).
Recently, Ohzuku et al, Chemistry Express, 7, 193 (1992), discovered a new type of LiMnO.sub.2 prepared from LiOH.multidot.H.sub.2 O and .gamma.-MnOOH at moderate temperature. Ohzuku et al. mixed stoichiometric amounts of the above reactants and then pressed them into a pellet. Different pellets were then heated in flowing nitrogen for several hours at temperatures between 300.degree. C. and 1000.degree. C. to prepare a series of materials. For materials prepared at 1000.degree. C., the powder diffraction pattern resembled that of orthorhombic LiMnO.sub.2, (Dittrich and Hoppe, Z. Anorg. Allg. Chemie 368, 262 (1969)). At lower temperature, (eg. 450.degree. C.) the patterns were different, consisting of Bragg peaks that could be assigned to LiMnO.sub.2 (of which some were very broad) and other peaks from an impurity phase. Nevertheless, Ohzuku et. al showed that material prepared at moderate temperature (between 300.degree. C. and 450.degree. C. apparently) reversibly reacted with about 190 mAh/gram of lithium between 2.0 and 4.25 V when operated as a lithium-ion cell cathode would be. In this application we call the material prepared at temperatures below 450.degree. C., (excluding the impurity phase) Low Temperature LiMnO.sub.2 or LT-LiMnO.sub.2.
Cobalt and nickel are both much more expensive than manganese. For this reason, it is very important to use manganese-based materials in a price sensitive battery product provided performance penalties compared to cobalt and nickel are not overly severe. The new material reported by Ohzuku et al. apparently represents a major improvement in specific capacity compared to LiMn.sub.2 O.sub.4. However, LiMn.sub.2 O.sub.4 delivers useable capacity at a higher voltage than LT-LiMnO.sub.2. Thus both materials appear very attractive for use as lithium ion battery cathodes.
Low cost, simple synthesis methods for such manganese based compounds are therefore desirable. Acta Chemica Sinica, Vol 39, No. 8, page 711-716 discusses an ion exchange material, LiMn.sub.2 O.sub.4, resulting from treatment of electrolytic MnO.sub.2 in LiOH solution followed by a heating stage. The preparation of LiMn.sub.2 O.sub.4 is also disclosed in U.S. Pat. No. 4,246,253 using a method involving sintering lithium carbonate and manganese dioxide at temperatures of 800.degree.-900.degree. C. It was shown in U.S. Pat. No. 4,828,834 that a preferred LiMn.sub.2 O.sub.4 compound for use in rechargeable batteries could be prepared by reacting manganese dioxide with Li.sub.2 CO.sub.3 at 400.degree. C. or with LiI at 300.degree. C. in nitrogen. Ohzuku et al. proposed a method for preparing LT-LiMnO.sub.2 which involves pelletizing an intimate mixture containing LiOH.multidot.H.sub.2 O and heat treating. In all these cases, manganese compounds with substantial amounts of lithium were prepared. However in all cases, an intimate mixture of lithium salt and manganese oxide must be created prior to heat treatment. The actual incorporation of lithium by ion exchange into the manganese oxide disclosed in Acta Chemica Sinica is presumably low since there are few hydrogen atoms to exchange with. However, after merely evaporating away water from LiOH solution, a solid intimate mix of LiOH coated, partially exchanged manganese oxide would be obtained. Upon heating such a mix, a solid state reaction could be expected to occur between the residual LiOH and the partially exchanged manganese oxide. Thus, substantial lithiation could be achieved via the solid state reaction and not by ion exchange.
There are several possible problems with synthesis methods employing such solid state reactions of intimate mixes of solids. In order to make uniformly lithiated material it is important that the stoichiometry throughout is constant. A uniform reaction relies on the correct ratios of reactants being present on a local, small scale. Thus, uniformity of the reactant mix must be achieved on a very small scale. Purity problems can arise if the manganese compound is inadequately lithiated prior to heating fully, resulting in the formation of an undesired compound. Unreacted lithium salt can remain as an impurity. In some instances, the preferred Li salt reactant may be LiOH. However, this can readily convert to Li.sub.2 CO.sub.3 in air. Thus exposure of the mixture to the normal atmosphere prior to finishing heat treatment may be undesirable. A method of dealing with this particular problem is disclosed in Japanese Kokai 04-115459, wherein Li.sub.2 CO.sub.3 is converted to LiOH by introducing water vapour into the process air stream.